I read a long thread on Vintage Mustang Forums, and the '65-'68 Mustang owners have also had problems with inaccurate gauge readings. I tried tweaking the float arm ever slightly to 73 ohms empty, and 10 ohms in the full position. This did absolutely nothing. I have to live with 3/4 on the gauge as full and 5 gallons left with the gauge reading totally E and the low fuel light constantly on. Someone commented on the forum that these reproductions from Taiwan have a linear resistor, so they'll never read accurately.
Installed earlier this year, and the the fuel gauge read less than 3/4 with a completely-full tank. Low fuel light was on with 4 gallons remaining, and gauge read completely empty. Darryl sent me a replacement in three days, but it's doing the exact same thing. The float arm could probably be bent to correct, but instruction is needed on how to do it without trial and error. Would not recommend this part.